The problem with "fast" service center is...
When submitted I-751 at WAC, the processing time was 7 months, then went all the way up to 14 or something. I ended up getting petition approved in 20.5 months.
It's not completely random, even if it seems. My friend who submitted I-751 two weeks later after me got it approved 4-5 months earlier than me. At that time all the cases around his were being processed. All the cases around mine were skipped for few months. Hard to tell what happened, but I think that's how USCIS tried "improving" processing times?